The execution of the North Korean leader Kim Jong-un's uncle is "an ominous sign" raising concerns about instability in a nation pursuing a nuclear arms drive, US Secretary of State John Kerry warned yesterday.

The execution of Jang Song-thaek, who was considered the second most powerful man in the secretive country, showed why the world must make a united stand against North Korea's nuclear weapons programme, Kerry said in an interview for US television.

The top US diplomat, on a trip to Vietnam, said the execution was not the first under Kim's rule.

"It tells us a significant amount about the instability internally of the regime, with the numbers of executions," Kerry said. "It's an ominous sign of the instability and of the danger that does exist."

Kerry said the execution showed the world "how ruthless and reckless" Kim was, and he likened him to the late Iraqi dictator Saddam Hussein.

"This is the nature of this ruthless, horrendous dictatorship and of his insecurities," he added.

The move also underscored the importance of trying to rein in the reclusive regime's nuclear ambitions through the stalled six-party talks aimed at denuclearising the Korean peninsula.

"To have a nuclear weapon, potentially, in the hands of somebody like Kim Jong-un becomes even more unacceptable," Kerry said.

The US was seeking to work with Pyongyang's close ally and neighbour China to find a way forward in the talks which have been on ice since 2008, he said.

"China is critical to any successful outcome with respect to denuclearising North Korea. And we are now doing a more cooperative approach to the peninsula," Kerry said.

Even though the outside world knew very little about the internal politics of the secretive regime, Kerry said Washington had gleaned a few insights about the young leader.

Kim was "spontaneous, erratic, still worried about his place in the power structure and manoeuvring to eliminate any potential kind of an adversary or competitor and does so, obviously, ruthlessly", Kerry said.

Kerry returned yesterday to Vietnam's Mekong Delta, which he navigated as a wartime gunboat skipper, to highlight its vulnerability to climate change.

He travelled by boat through Ca Mau, a once-dangerous Viet Cong stronghold. At the pier of the small port of Kien Vang, he spoke to officials and students. "It is obviously amazing for me to be here today," he said.

"Decades ago, on these very waters, I was one of many who witnessed the difficult period in our shared history. Today... I'm bearing witness to how far our nations have come together."

Additional reporting by Reuters

This article appeared in the South China Morning Post print edition as Execution 'ominous sign', Kerry says

"It tells us a significant amount about the instability internally of the regime, with the numbers of executions," Kerry said. "It's an ominous sign of the instability and of the danger that does exist."Hang on a second - According to Amnesty International, in 2012 there were 6 executions in North Korea; 43 in the United States.